214 University of California Publications in Geology [Vol. 13 



silicate minerals. Above the upper level the ore has been completely 

 oxidized, the residue consisting of limonite and the silicate minerals 

 of the gangue. In a zone along the contact between completely oxi- 

 dized and partially oxidized ore, sulphates and arsenates of nickel and 

 cobalt are present, including erythrite (hydrous cobalt arsenate, 

 "cobalt bloom") and morenosite (hydrous nickel sulphate). Below 

 the completely oxidized material on the upper level the sulphides have 

 been decomposed along fracture planes and there is a slight amount 

 of oxidation even in the ore of the lower level. 



127 ft. level 



170 ft. level 



Fig. 2. Vertical section through the Friday Mine ore body. 

 . Scale: 1 inch =30 feet. 



Basic dikes. — Narrow dikes of fine-grained hornblende gabbro and 

 hornblende norite cut the massive gabbroic rocks at several localities. 

 Near the bottom of the winze on the upper level one of these dikes cuts 

 the sulphide ore. All the dikes show marked similarity among them- 

 selves, both in mineral composition and grain. They probably repre- 

 sent a late stage of the activity of the Cuyamaca Basic Intrusive 

 magma. The fact that the ore is cut by a dike of this type is evidence 

 of a close relationship between ore formation and magmatic processes. 



Pegmatite. — A narrow dike of pegmatite cuts gabbroic rock 77 feet 

 east of the shaft on the upper level. On the lower level the pegma- 

 tite is much more abundant. It occurs as narrow dikes cutting the 

 basic igneous rocks at several places, and also as a large irregular 

 mass, cutting both norite and schist, at a point near the junction of 

 the east-west drift and the north cross-cut. At the latter place the 

 pegmatite is particularly coarse, carrying feldspars several inches in 

 diameter and tourmalines up to 6 inches in length. The pegmatite 



